See also

In order to stop or restart the Apache HTTP Server, you must send a signal to
the running httpd processes. There are two ways to
send the signals. First, you can use the unix kill
command to directly send signals to the processes. You will
notice many httpd executables running on your system,
but you should not send signals to any of them except the parent,
whose pid is in the PidFile. That is to say you
shouldn't ever need to send signals to any process except the
parent. There are four signals that you can send the parent:
TERM,
USR1,
HUP, and
WINCH, which
will be described in a moment.

To send a signal to the parent you should issue a command
such as:

kill -TERM `cat /usr/local/apache2/logs/httpd.pid`

The second method of signaling the httpd processes
is to use the -k command line options: stop,
restart, graceful and graceful-stop,
as described below. These are arguments to the httpd binary, but we recommend that
you send them using the apachectl control script, which
will pass them through to httpd.

After you have signaled httpd, you can read about
its progress by issuing:

Sending the TERM or stop signal to
the parent causes it to immediately attempt to kill off all of its
children. It may take it several seconds to complete killing off
its children. Then the parent itself exits. Any requests in
progress are terminated, and no further requests are served.

The USR1 or graceful signal causes
the parent process to advise the children to exit after
their current request (or to exit immediately if they're not
serving anything). The parent re-reads its configuration files and
re-opens its log files. As each child dies off the parent replaces
it with a child from the new generation of the
configuration, which begins serving new requests immediately.

This code is designed to always respect the process control
directive of the MPMs, so the number of processes and threads
available to serve clients will be maintained at the appropriate
values throughout the restart process. Furthermore, it respects
StartServers in the
following manner: if after one second at least StartServers new children have not
been created, then create enough to pick up the slack. Hence the
code tries to maintain both the number of children appropriate for
the current load on the server, and respect your wishes with the
StartServers
parameter.

Users of mod_status
will notice that the server statistics are not
set to zero when a USR1 is sent. The code was
written to both minimize the time in which the server is unable
to serve new requests (they will be queued up by the operating
system, so they're not lost in any event) and to respect your
tuning parameters. In order to do this it has to keep the
scoreboard used to keep track of all children across
generations.

The status module will also use a G to indicate
those children which are still serving requests started before
the graceful restart was given.

At present there is no way for a log rotation script using
USR1 to know for certain that all children writing
the pre-restart log have finished. We suggest that you use a
suitable delay after sending the USR1 signal
before you do anything with the old log. For example if most of
your hits take less than 10 minutes to complete for users on
low bandwidth links then you could wait 15 minutes before doing
anything with the old log.

When you issue a restart, a syntax check is first run, to
ensure that there are no errors in the configuration files.
If your configuration file has errors in it, you will get an
error message about that syntax error, and the server will refuse to
restart. This avoids the situation where the server halts and then
cannot restart, leaving you with a non-functioning server.

This still will not
guarantee that the server will restart correctly. To check the
semantics of the configuration files as well as the syntax, you
can try starting httpd as a non-root user. If there
are no errors it will attempt to open its sockets and logs and fail
because it's not root (or because the currently running
httpd already has those ports bound). If it fails
for any other reason then it's probably a config file error and the error
should be fixed before issuing the graceful restart.

Sending the HUP or restart signal to
the parent causes it to kill off its children like in
TERM, but the parent doesn't exit. It re-reads its
configuration files, and re-opens any log files. Then it spawns a
new set of children and continues serving hits.

Users of mod_status
will notice that the server statistics are set to zero when a
HUP is sent.

As with a graceful restart, a syntax check is run before the
restart is attempted. If your configuration file has errors in it, the
restart will not be attempted, and you will receive notification of the
syntax error(s).

The WINCH or graceful-stop signal causes
the parent process to advise the children to exit after
their current request (or to exit immediately if they're not
serving anything). The parent will then remove its PidFile and cease listening on
all ports. The parent will continue to run, and monitor children
which are handling requests. Once all children have finalised
and exited or the timeout specified by the GracefulShutdownTimeout has been
reached, the parent will also exit. If the timeout is reached,
any remaining children will be sent the TERM signal
to force them to exit.

A TERM signal will immediately terminate the
parent process and all children when in the "graceful" state. However
as the PidFile will
have been removed, you will not be able to use
apachectl or httpd to send this signal.

The graceful-stop signal allows you to run multiple
identically configured instances of httpd at the
same time. This is a powerful feature when performing graceful
upgrades of httpd, however it can also cause deadlocks and race
conditions with some configurations.

Care has been taken to ensure that on-disk files such as lock files
(Mutex) and Unix socket files
(ScriptSock) contain the server
PID, and should coexist without problem. However, if a configuration
directive, third-party module or persistent CGI utilises any other on-disk
lock or state files, care should be taken to ensure that multiple running
instances of httpd do not clobber each other's files.

You should also be wary of other potential race conditions, such as
using rotatelogs style piped logging. Multiple running
instances of rotatelogs attempting to rotate the same
logfiles at the same time may destroy each other's logfiles.

Notice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.